In very high voltage power systems such as those handling 500 kV and above, proper insulation between different units of the system is important so as to ensure the safe operation thereof.
As one example, the connection between power transformers and bushings, which connection provides a high voltage interface between e.g. a power grid and the power transformer must be insulated for protection against for instance arcing which may occur due to the high voltages involved. Thereby the risk of damaging the power transformer or any other power equipment in the vicinity of the power transformer is reduced.
A bushing guides a high voltage conductor from outside the power transformer through the grounded power transformer housing, whereby the conductor is connected to a winding of a leg of the power transformer core contained in the housing.
In the transformer and in the bushing, typically transformer oil or similar dielectric fluid or gas is provided so as to insulate the power transformer and the bushing.
Typically the bushing conductor and transformer conductor connection is shielded for reducing the stress attained in this area due to the very high electric fields which are present.
WO 2007/111564 discloses a high voltage insulation system where the connection between the bushing conductor and the transformer conductor is shielded by means of a shielding electrode. Several insulation barriers are fastened to the shielding electrode so as to subdivide the oil volume around the electrode. Thereby the concentration of voltage stress due to the restriction of the current flow induced by inter alia the insulation barriers, is divided between the shielding electrode and a cylindrical barrier of the insulating system.
A drawback with the solution presented in WO 2007/111564 is that it may increase the electric field stress in the insulation system.